Saving should be no problem, but maybe if it’s a color image there may be issues. Could you try if saving works if you convert the image to scalar (grayscale) volume (using Vector to scalar volume module)?
Mutual information minimizes the entropy of the joint histogram. You can find the final value in the application log (the metric value should decrease in each iteration), but it’s not a metric of registration quality.
Registration is typically assessed using spatial error metrics. If you have ground truth landmark points then compute TRE. You may also compute Hausdorff distance using Segment Comparison module (between ground truth segmentation in the fixed image and segmentation in moving image transformed to fixed image coordinate system).
The only intensity-based metric that I’m aware of that might be somewhat applicable for assessing registration (or, more accurately, the effect of remaining registration error) is the module for comparison of dose volumes (in SlicerRT extension).